Vertical pump natural frequency
Vertical pump natural frequency
(OP)
We have three vertical pumps installed on cooling tower. Pumps are supplied by Flowserve. During commissioning we faced a problem with tripping on high vibration during start-up. We have measured on natural frequency on the casing of the pump and motor (bump test). It is 5.41 Hz whereas the running speed frequency is 7 Hz. Shouldn't the natural frequency be above the running speed frequency?





RE: Vertical pump natural frequency
We have some vertical motors which we bump test while secured. In this configuration all of the water drains out of the casing. I have often wondered if the actual natural frequency during running will be lower than measured due to additional water mass present while running (missing during bump). In your case of measured bump test below natural frequency this is not a concern.
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RE: Vertical pump natural frequency
Structures comprising beams designed to "deflection as a percent of span" rules will have a natural frequency around 6 Hz.
A small stepping motor, attached rigidly at the pump mount, swept slowly through the frequency band from 4..8 Hz might provide some surprises. It doesn't even need an unbalanced load.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Vertical pump natural frequency
Skip Hartman
http://www.machinerywatch.com
RE: Vertical pump natural frequency
In summary, set a higher trip limit for the startup, and then use the normal trip units once the unit is up to speed. If your system is not programmable, then press the "reset" button after startup, and use the normal trip limits.
RE: Vertical pump natural frequency
After conducting change-over test on the pumps when we started stand-by pump and stoped running one we experianced tripping of already running pump on high vibration when stand-by one starts. So I think not only natural frequency is contributing to this problem but may be the valve arrangement on the discharge of the pumps as the pumps work to common header through butterfly valves at each pump discharge (no check valve).